1. Field of the Invention
This invention relates to a process for preparing magnetic iron oxide, and more specifically, to a process for preparing improved magnetic iron oxide by doping Co into the iron oxide and increasing the magnetic anisotropy of the resulting magnetic iron oxide powder having a large coercive force.
2. Description of the Prior Art
Production of high density magnetic recording materials requires magnetic materials having a large coercive force and a high squareness ratio (the ratio between the residual magnetic flux and the saturated or maximum magnetic flux).
In order to increase the coercive force of a magnetic powder of the iron oxide type, it is effective to dissolve Co in the magnetic powder, and various methods for achieving this have been proposed in the past, for example, as disclosed in U.S. Pat. No. 3,117,933, U.S. Pat. No. 3,671,435, Japanese Patent Publication No. 6,113/67, Japanese Patent Publication No. 27,719/66 (corresponding to U.S. Pat. No. 3,573,980), Japanese Patent Publication No. 15,759/73, and Japanese Patent Publication No. 10,994/73.
In order to obtain a high squareness ratio, it is known to orient the needle-like particles using a magnetic field orienting treatment while the coating composition containing the particles is still in the form of a coating or before the coating composition has been dried, and in the case of Co-containing iron oxide, especially iron oxide of the magnetite type containing Fe.sup.++ ion, it is known the squareness ratio can be improved by a magnetic annealing treatment in a magnetic field. Methods utilizing this approach are disclosed, for example, in Japanese Patent Publication No. 18,282/61 (corresponding to British Pat. No. 951,104), Japanese Patent Publication No. 5,927/61 (corresponding to U.S. Pat. No. 3,026,215), and Japanese Patent Publication No. 17,113/64.
The present invention relates to a further improvement of these prior techniques, and relates to an improvement of the magnetic annealing treatment mentioned above.